1. Field of the Invention
The present invention relates to an optical recording medium including a plurality of recording and reading layers, an optical recording and reading method and an optical recording and reading apparatus that record information on the optical recording medium, and a method for producing the optical recording medium.
2. Description of the Related Art
Conventionally, optical recording media such as a CD-DA, a CD-ROM, a CD-R, a CD-RW, a DVD-ROM, a DVD-R, a DVD+/−RW, a DVD-RAM and a Blu-ray Disc (BD) have been widely used for viewing digital moving image content or recording digital data. Among them, in the BD, which is regarded as one of next-generation DVD standards, a wavelength of laser light used for recording and reading is set to be short, that is, 405 nm, and a numerical aperture of an objective lens is set to 0.85. In an optical recording medium conforming to the BD standard, tracks are formed at a pitch of 0.32 μm. In this way, data of 25 GB or more can be recorded/read on/from one recording and reading layer of the optical recording medium.
Incidentally, capacities of moving images and data are expected to increase more and more in the future. Accordingly, there has been considered a method for increasing a capacity of the optical recording medium by providing multiple recording and reading layers in the optical recording medium. For the optical recording medium according to the BD standard, there has also been reported a technique for realizing an ultra-high capacity of as much as 200 GB by providing six to eight recording and reading layers (see I. Ichimura et. al., Appl. Opt, 45, 1974-1803 (2006), and K. Mishima et. al., Proc. of SPIE, 6282, 62820I (2006)).
On the contrary, if the multiple recording and reading layers are provided in the optical recording medium, formation of concavo-convex patterns such as grooves/lands for tracking control on each recording and reading layer leads to a complicated medium configuration, causing concern about difficult eccentricity adjustment work and the like. Moreover, a stamper as a mother die for forming the concavo-convex patterns is required for each provision of the recording and reading layer. Thus, a larger number of layers increases the number of times of use of this stamper, and accordingly increases production costs.
Consequently, in recent years, there has been proposed a technique for separately providing servo layers having concavo-convex patterns and grooves, and recording and reading layers not having the concavo-convex patterns and the grooves, in an optical recording medium, and recording information on the recording and reading layers by using a beam dedicated to recording and reading, while obtaining tracking signals from the servo layers by using a beam dedicated to tracking control (see Japanese Patent Laid-Open Nos. 2008-97693 and 2008-97694).
More recently, there have been proposed techniques of a 20-layered ROM type optical recording medium (see A. Mitsumori et. al., Jpn. J. Appl. Phys., 48, 03A055 (2009)), a 10 to 16-layered write-once optical recording medium (see T. Kikukawa et. al., Jpn. J. Appl. Phys., 49, 08KF01 (2010), M. Inoue et. al., Proc. SPIE, 7730, 77300D (2010), and M. Ogasawara et. al., Tech. Dig. of International Symposium on Optical Memory 2010, 224 (2010)), and the like. Use of an optical system (a wavelength and a numerical aperture) similar to the BD standard has become likely to be able to achieve a recording capacity on the order of 500 GB.
Moreover, for the purpose of simultaneously reading two layers, there has also been proposed a technique for alternately stacking recording layers having concavo-convex patterns and grooves, and recording layers not having the concavo-convex patterns and the grooves (see International Publication No. WO2008/099708).
However, according to the technique of International Publication No. WO2008/099708, an increase in the number of the recording and reading layers causes the number of the recording layers having the concavo-convex patterns and the grooves to increase, which, after all, leads to the complicated medium configuration, the difficult eccentricity adjustment work and the like.
Moreover, according to the techniques of Japanese Patent Laid-Open Nos. 2008-97693 and 2008-97694, as the number of the recording and reading layers increases, a distance between the recording and reading layer and the servo layer increases, and thus a thickness tolerance can easily occur between both layers. Specifically, the recording and reading beam applied to the recording and reading layers performs focus control, and the tracking beam applied to the servo layers performs only tracking control. In other words, a focus of the tracking beam depends on the focus control performed by the recording and reading beam. As a result, when the thickness tolerance becomes large, a focus position of the tracking beam varies to destabilize the tracking signal, which causes a problem that sufficient tracking control cannot be performed. In order to solve this problem, it is conceivable to introduce a focus servo mechanism also into an optical system of the beam performing the tracking control. Correction of the thickness tolerance is, however, limited, and the introduction of the focus servo mechanism also causes a problem of a large optical pickup.
In particular, if the recording and reading beam and the tracking beam share one objective lens, the focus positions of the both beams need to be shifted from each other by setting different spread angles for the both beams being caused to enter the objective lens. As the difference between the spread angles of the both beams becomes larger, a shift between the tracking control performed by the tracking beam and the tracking control required for the recording and reading beam becomes larger. For example, there has been a problem that a tracking position is greatly shifted from a recording position on a warped optical recording medium or the like.
Moreover, as the number of the recording and reading layers on the optical recording medium is increased, a distance for moving a recording and reading beam spot into a stacking direction increases. As a result, there has been a disadvantageous problem with comatic aberration caused by a tilt or the like in recording and reading.
Moreover, in the case of the techniques of Japanese Patent Laid-Open Nos. 2008-97693 and 2008-97694, as well as International Publication No. WO2008/099708, for example, there has been a problem that, when the beam dedicated to recording and reading passes through a desired recording and reading layer, this leakage light is reflected by the servo layer to cause crosstalk. In particular, when the number of the recording and reading layers is increased, a reflectance of each recording and reading layer must be reduced. Thus, there has been a problem of an increase in an amount of the leakage of the beam dedicated to recording and reading.
Furthermore, when the number of the recording and reading layers is increased, the recording and reading layers are disposed over a wide range in a thickness direction in the optical recording medium. As a result, a recording and reading optical pickup is required to focus the beam on the wide range in the thickness direction. Thus, a large spherical aberration correction range must be set. Accordingly, there has been a problem of a complicated and larger configuration of the optical pickup, and also, a long seek time for the recording and reading layers with the optical pickup.
Moreover, the increase in the number of the recording and reading layers causes the capacity of the optical recording medium to increase, which, however, does not solely lead to improvement in a recording and reading speed. For example, if the recording capacity of the optical recording medium increases without the improvement in the recording speed, there has been a problem that a user has to wait longer in recording work and feels reduction in convenience.
Furthermore, if information is recorded on an optical recording medium including a plurality of recording and reading layers, OPC (Optimum Power Control) for optimizing recording laser power and its output parameter needs to be performed for each recording and reading layer. It should be noted that, according to this OPC, a recording power level (Pw), an erasing power level (Pe) and the like of a laser are optimized by recording random data in a trial writing area of each recording and reading layer while varying output power in a stepwise manner, and then reproducing and analyzing this recorded data. With employment of the OPC, the laser power can be optimized immediately before recording, in consideration of a usage environment such as a temperature, an individual difference in a laser included in a drive, degradation of each recording and reading layer over time, and the like. However, there has been a problem of an increase in a preparation time before recording, for this optimization.
In particular, when information is continuously recorded across the plurality of recording and reading layers, also in order to maintain continuity of an information transfer rate, a preparatory operation is required for previously performing the OPC for all the plurality of recording and reading layers that are recording targets, and retaining different output parameters for the respective recording and reading layers, in a storage memory. As a result, there has been a problem of a further increase in the preparation time before recording.
Moreover, in the OPC, since the random data is recorded while the output power is varied in a stepwise manner, an extraordinarily high power beam is applied to the trial writing area to form an abnormal mark. For example, while the OPC is performed for a trial writing area of one particular recording and reading layer, if an abnormal mark has been formed in a trial writing area of a recording and reading layer adjacent to this layer, noticeable reflection noise is caused by this abnormal mark so that correct OPC cannot be performed. In order to eliminate this problem, a larger trial writing area must be prepared so that the random data to be recorded in the OPC is controlled so as not to overlap between the adjacent recording and reading layers. As a result, there has been a problem that, as the number of the recording and reading layers is increased for the purpose of increasing the capacity of the optical recording medium, the trial writing area of each recording and reading layer must be larger and a user data area is decreased.
As in Japanese Patent Laid-Open Nos. 2008-97693 and 2008-97694, as well as International Publication No. WO2008/099708, in the optical recording medium provided with the servo layers and the recording and reading layers separately, a shift in a radial direction can easily occur at a recording mark forming position on the recording and reading layer farther from the servo layer. Accordingly, there has been a problem that the larger trial writing area also needs to be secured in anticipation of this shift, and the user data area is further decreased. Moreover, for example, in order to increase the recording and reading layers, when a plurality of servo layers are formed on one plane side of the optical recording medium, internal stress caused in film formation can easily incline toward one side of the optical recording medium, and thus warpage or distortion occurs on the optical recording medium. Since the shift in the radial direction can more easily occur at the recording mark forming position, also due to this warpage or distortion, there has been a problem that a further larger trial writing area must be secured.
According to the techniques of Japanese Patent Laid-Open Nos. 2008-97693 and 2008-97694, there is a process of alternately stacking the recording and reading layers and spacer layers, in production of the optical recording medium. Thus, the internal stress caused in each film formation inclines toward one plane of the optical recording medium. As a result, there has been a problem that large warpage or distortion can easily occur on the optical recording medium. In particular, the increase in the number of the recording and reading layers causes an amount of accumulated internal stress to increase, and also causes an amount of deformation of the optical recording medium to increase. There has been a problem that the deformation of the optical recording medium causes a tracking beam spot to be shifted from the recording and reading beam spot, and causes a tracking property to be significantly degraded.
As seen from the above, as in the techniques of Japanese Patent Laid-Open Nos. 2008-97693 and 2008-97694, the optical recording medium provided with the servo layers and the recording and reading layers separately is required to further reduce the warpage or the deformation amount of the medium. In order to reduce the warpage or the deformation amount, it is also conceivable to use an ultra-rigid body such as a glass substrate, which, however, increases the production costs.
Furthermore, if the plurality of servo layers are formed in one optical recording medium, positions of the grooves/lands of the respective servo layers may be shifted from one another. In this case, a relative position of a recording mark to be formed on the recording and reading layer differs depending on which servo layer has been used for recording. In such a situation, for example, complicated control is required to simultaneously record information on two recording and reading layers, or to simultaneously reproduce data recorded on the two recording and reading layers.